1. Field of the Invention
This invention relates to sewing machines in general and more particularly to a detector for advising a sewing machine operator of the impending depletion of bobbin thread.
2. Description of the Prior Art
Bobbin thread run-out detectors for warning a sewing machine operator of the impending exhaustion of bobbin thread are known in the prior art. Some forms of prior detectors utilize a light source which is aimed across a thread-carrying bobbin toward a photodetector located on the opposite side of the bobbin. When the bobbin contains a quantity of thread, the light is blocked from illuminating the photodetector. The photodetector is exposed to light only after the quantity of thread wound on the bobbin has been depleted during the sewing process. Still other prior bobbin thread run-out detectors have employed a bobbin having a core containing a plurality of reflecting surfaces and a light source and photodetector located on the same side of the bobbin. Light from the source is aimed at the reflective bobbin core, and is scattered as long as bobbin thread remains wrapped around the core. When the core is exposed by the depletion of bobbin thread, the light is reflected from the core toward the photodetector which operates a device to warn the operator of the impending exhaustion of bobbin thread. Bobbin thread run-out detectors are also known in which the bobbin is constructed with an annular thread storage flange located between the end flanges. The thread storage flange forms a storage cavity in which the last remaining wraps of bobbin thread are contained. A light source and photodetector are placed so that the bobbin is interposed therebetween, with the optical axis joining the light source and photodetector offset from the axis of rotation of the bobbin, but nevertheless contained within the radius of the storage flange. If the bobbin is properly wound with thread, the thread contained within the storage cavity will not be consumed until all other thread is removed from the bobbin during the sewing process. As thread is removed from the storage cavity the optical path between the light source and the photodetector will be exposed, thereby allowing light to impinge on the photodetector, which may activate a device to warn the sewing machine operator of the impending depletion of bobbin thread.
One problem with some known sewing machine bobbin thread run-out detectors is that they require a light source and photodetector placed at opposite sides of the loop taker, in areas which are crowded with other sewing machine instrumentalities.
Another problem is that the sensitivety and reliability of some bobbin thread run-out detectors vary with the age, voltage, and temperature of the constituent components.
Still another problem is that some prior known bobbin thread run-out detectors could not be adjusted to activate an alarm when a variable quantity of thread remained on the bobbin.
Another problem with some known bobbin thread run-out detectors is that the light source and photodetector are located in areas which are susceptible to impaired operation due to lint and fabric debris which collect in the loop taker cavity.
Still another problem is that bobbin thread run-out detectors which utilize bobbins having a thread storage flange for segregating the last wraps of bobbin thread must be properly filled with thread in a special manner to operate properly.
Another problem is that bobbin thread run-out detectors which have a light source on one side of the bobbin and the photodetector on the other side require that apertures be machined in the bobbin case for the passage of light therethrough.
Still another problem is that some known bobbin thread run-out detectors are susceptible to false readings due to the presence of stray light in the area surrounding the photodetector.